Method for producing a composite shadow mask

ABSTRACT

A mesh electrode is coated with an electrically insulating material. The portions of the insulating material are removed where the beam apertures of a shadow mask are to be located when the coated mesh electrode and the shadow mask are combined together. And after finishing the partial removal of the insulating material, the mesh electrode is combined with the shadow mask to produce a composite shadow mask having a converging action upon the electron beam.

United States Patent Oikawa et al.

[ 1 Sept. 5, 1972 METHOD FOR PRODUCING A COMPOSITE SHADOW MASK Inventors: Mitsuru Oikawa, Tokyo; Shozo Tamura, Hachioji; Tadao Okabe, Hachioji, all of Japan Assignee: Hatachi, Ltd., Tokyo, Japan Filed: Sept. 1, 1970 Appl. No.: 68,601

Foreign Application Priority Data Sept. 5, 1969 Japan ..44/7000l US. Cl ..29/25.14, 29/2513 Int. Cl ..H0lj 9/00 Field of Search ..29/25.l, 25.11, 25.13, 25.14, 29/25.l7, 25.18; 313/85 S, 92 B References Cited UNITED STATES PATENTS 2,874,449 2/1959 De Rooy et a1 ..29/25.14 3,474,511 10/1969 Ort et a1. ..29/25.l4

Primary ExaminerJohn F. Campbell Assistant Examiner-Richard Bernard Lazarus Attorney-Craig, Antonelli, Stewart & Hill [57] ABSTRACT 2 Claims, 2 Drawing Figures PATENTEDsEP 5 m2 FIG. 2

INVENTORS MH'SLLRKL (MKRWH SHOZO TRMLXRR THDRO OKHBE ATR )RNEYS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a shadow mask having a converging action upon an electron beam, and more particularly to a method for fabricating a composite shadow mask in which two electrodes, one of which is formed of a mask-like electrical conductor, are superposed with an insulating material interposed therebetween, between which a slight voltage is impressed so that electron lenses may be formed at the beam apertures of a resulted shadow mask.

2. Description of the Prior Art In general, the factor of electron beam utilization of a color picture tube provided with a shadow mask has been smaller than that of a black-and-white picture tube which has no shadow mask because of the provision of the shadow mask. And this has caused an inferior brightness in the color picture tube as compared to the black-and-white picture tube. In order to improve the brightness of the color picture tube, it is necessary to enlarge each beam aperture in the shadow mask so as to improve the utilization of the electron beam. When each of the beam apertures is enlarged, it becomes necessary to strongly converge or focus the electron beam after it has passed through the shadow mask so as to cause the beam to impinge accurately on each of the color phosphor dots distributed on the screen of the color picture tube. In order to fulfill these requirements, there have been proposed a shadow mask in which two electrodes are disposed adjacent to each other, between which a slight voltage is impressed so that each beam aperture of the shadow mask may serve as an electron lens. One of these electrodes may be a shadow mask electrode having the same construc tion as that of a conventional shadow mask and the other is a shadow mask lens electrode comprising a mesh electrode or an electrode having a great number of tiny holes which forms with the shadow mask electrode an electron lens. The potential given to the shadow mask electrode is slightly lower than that given to the shadow mask lens electrode. With this construction each beam aperture of the shadow mask constitutes an electrostatic lens which converges the electron beam which passes through the aperture toward the center of the beam aperture. Thus, the dimension of the cross section of the electron beam impringing upon the phosphor screen is rendered much smaller than that of the beam aperture of the shadow mask electrode.

The use of a shadow mask having such a construction as described above can add to the improvement not only in the beam utilization factor, i.e. the brightness of picture reproduced, but also to an improvement in color purity and resolution.

In this way the shadow mask has several advantages as described above, but it is extremely difficult from the practical fabricating point of view to insert an insulating material between the shadow mask electrode and the shadow mask lens electrode, in particular the mesh like conductive electrode, without filling up some of the many beam apertures of the shadow mask with insulating material. Moreover, the electrification of the insulating material also adversely affects the electron beam, thus making difficult the realization of the proposed shadow mask.

SUMMARY OF THE INVENTION shadow mask having converging action upon electron beam. According to this invention this object is attained by relatively few steps. Namely, a shadow mask lens electrode in the form of a mesh electrode is coated with an insulating material, then the portions of the coating insulating material are removed from the shadow mask lens electrode where the beam apertures of a conventional shadow mask will lie when the coated lens electrode and the shadow mask electrode are combined together, and finally the shadow mask electrode and the shadow mask lens electrode are combined together after the partial removal of the insulating material to produce a shadow mask so designed as to converge the electron beam passing therethrough.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross sectional view of a shadow mask made according to this invention, which will have a converging action upon electron beam, and

FIG. 2 is a plan view taken along the line IIII of FIG. 1.

In the figures, numeral 1 shows a shadow mask electrode; a shadow mask lens electrode is designated at 2; an insulating material layer 3 is provided over said shadow mask lens electrode 2; and numerals 4 and 5 indicate beam apertures provided respectively in said shadow mask electrode 1 and said shadow mask lens electrode 2.

The composite shadow mask as described above is disposed adjacent to the phosphor dot screen (not shown) between the phosphor dot screen and the electron gun (not shown) and a voltage, for example, of l KV is applied between said shadow mask electrode 1 and said shadow mask lens electrode 2 in the conventional manner. Then, every beam aperture (constituted by beam apertures 4 and 5) of the shadow mask (constituted by shadow mask electrode 1 and shadow mask lens electrode 2 and hereafter referred to as composite shadow mask) forms an electron lens. And the electron lens thus formed acts to converge the electron beam passing through the beam aperture toward the center axis of the aperture. The insulating layer 3 serves to support said shadow mask lens electrode 2 on said shadow mask electrode 1 in an insulated condition. The lens electrode 2 may be formed of a mesh of electric conductors, i.e., a woven, knitted, expanded metal, or a metal plate having a great number of holes, the interstitial dimensions or the hole dimensions of which are small compared to the aperture dimensions of the shadow mask electrode.

Next, a method for fabricating such a composite shadow mask as described above will be explained in the following. In the embodiment described in this specification, the explanation is limited to a composite shadow mask using a mesh conductor as a shadow mask lens electrode. However, it is apparent that the present invention is not limited to the embodiment alone, but that some other substitutions, modifications and alternations are possible without departing from the scope of this invention.

The first step of fabricating the composite shadow mask is to coat a mesh conductor with an insulating layer such as made of, for example, glass. Such a mesh conductor is, for example, constructed with stainless steel or tungsten wire having a diameter of the mesh conductor comprising 230 meshes. Then, a paste like glass is prepared by dispersing powder of glass into a dispersant, such as nitrocellulose. The mesh conductor is immersed into the paste like glass so that the former is covered with the latter. Thereafter, the mesh conductor with liquid glass coating is dried and the glass powder coating remains over the mesh conductor. A desirable material used for the glass powder may be, for example, an amorphous inorganic substance having a high specific resistance and the softening temperature thereof must be higher than 450C at which the sealing of the picture tube glass envelope is performed. For example, silicate containing a major part of lead oxide, boron oxide or zinc oxide is preferable. The mesh conductor having glass powder coating thereover is subjected to heat treatment at temperatures 550 600C in an electric furnace so that the glass powder fuses. In this way, the structure for the shadow mask lens electrode is fabricated in which the mesh conductor is completely embedded in the glass insulating layer.

In the second step of fabricating the composite shadow mask, the thus furnished structure is set on a mold having the same radius of curvature as its respective shadow mask electrode, then the structure is pressed into a desired shape while the glass insulating layer is maintained at its softening temperatures, and finally the shaped structure is mounted on and fixed to a mechanically tough frame so designed as to combine it with the associated shadow mask electrode. Further heating and molding is to be carried out after the mounting of the structure on the frame, if desired.

The third step of fabricating the composite shadow mask is carried on as follows. A photoresist material which is soluble in an appropriate solvent when exposed to light, is applied to one or both side surfaces of the shadow mask lens electrode structure. Then, the lens electrode structure treated with the photoresist material is exposed to light with the shadow mask electrode masking it from the light. Accordingly, only the portions of the photoresist material lying beneath the beam apertures are exposed to the light and therefore those portions can be removed by a suitable developing treatment using such a solvent as aforementioned. It is preferable, in order to achieve the exposure with a great accuracy, that a d.c. voltage of about 1 KV should be applied between the shadow mask electrode and the shadow mask lens electrode during exposure so as to cause both the electrodes to be attracted to each other. In this case, the electrostatic attraction between the electrodes does not bring about such an adverse result, for example, as cracks in the glass coating on the mesh conductor, i.e., the shadow mask lens lens electrode, since the shadow mask electrode and the lens electrode structure are kept in close contact with each other by means of the combining frame.

According to the fourth step of fabricating the composite shadow mask, the shadow mask lens electrode structure after the developing treatment in the third step is first treated with an erodent such as fluoric acid or nitric acid so that the glass coating on the mesh con ductor, i.e. the shadow mask lens electrode is partially removed where the light hit the lens electrode structure through the beam apertures of the shadow mask electrode during exposure. The mesh conductor, i.e. the shadow mask lens electrode is made of stainless steel or tungsten as described above and is not dissolved in the erodent such as fluoric acid or nitric acid which dissolves the glass coating. Moreover, by appropriately setting the time of reaction, it is possible to make each eroded area corresponding to each beam aperture of the shadow mask electrode of the glass coating larger than that of the corresponding beam aperture, hence to eliminate the electrification of the glass insulating layer due to the electron beam striking the laye r Now, the shadow mask lens electrode structure thus prepared is combined with the associated shadow mask electrode so that a composite shadow mask having a converging action upon the electron beam may be produced. In the above embodiment, the shadow mask lens electrode structure and its associate shadow mask electrode are arranged to be kept in close contact with each other. Of course, it is possible, if necessary, to place them separately at a predetermined distance. In such a case, it is noted that the lens electrode structure and shadow mask electrode should be disposed separately at the same distance as mentioned above also when subjected to the exposing treatment.

As described in detail hereinbefore, according to the present invention, the shadow mask lens electrode is coated with an insulating glass and no specific insulation layer between the lens electrode and the shadow mask electrode is necessary. This will add to simplicity in fabricating a composite shadow mask of the type as described above. If an insulating layer is provided on the shadow mask electrode, as is the case with the conventional composite shadow mask, then the insulating layer will be broken on account of temperature variations since the thermal expansion of the insulating layer is different from that of the shadow mask electrode. The composite shadow mask according to the present invention never suffers from such a problem relating to the different coefficients of thermal expansion. Moreover, according to the present method, the diameter of each beam aperture of the shadow mask lens electrode structure, i.e. each eroded area corresponding to each beam aperture of the shadow mask electrode can be chosen large enough as desired by controlling, for example, the time of erosion. The collision of the electron beam against the insulation layer can therefore be eliminated so that the layer is free from being electrified. This fact will also add to the merits of this invention.

We claim:

1. A method for fabricating a composite shadow mask having a shadow mask electrode and a mesh like shadow mask lens electrode covered with an insulating layer except for portions corresponding to aperture portions of the shadow mask electrode, said method comprising the steps of: coating a mesh like electrical conductor used for the shadow mask lens electrode with a paste of glass-like material which is prepared by dispersing in a dispersant an inorganic material having a high specific resistance; heating said glass-like material to a temperature at which said glass-like material is fused and drying the same to make the insulating layer thereby forming a shadow mask lens electrode structure; moulding said lens electrode structure under heat to shape the same equally to the shape of said shadow mask electrode with which the lens electrode structure is to be associated; applying onto at least one of the two surfaces of said lens electrode structure a photoresist agent which is soluble in an appropriate solvent when exposed to light; exposing said lens electrode structure with said photoresist agent to light through said shadow mask electrode used for a mask for the exposure and subjecting the exposed lens electrode structure to developing treatment to remove the exposed portions of the photoresist agent; dissolving by an erodent portions of said insulating layer corresponding to the UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 688, 359 Dated September 5, 1972 Inventor(s) Mitsuru Oikawa, et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Name of assignee misspelled. Should read:

--Hitachi, Ltd., Tokyo, Japan-- Signed and sealed this 15th day of October 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents ORM uoss) uscoMM-Dc dean-P09 i ,5. GOVERNMENT PRINTING OFFICE 2 9'9 O-SB-SJL 

2. A method for fabricating a composite shadow mask according to claim 1 wherein in said exposing step a direct current voltage is applied between said shadow mask electrode and said lens electrode so as to cause both the electrodes to be attracted to each other. 